(1) Technical Field
This invention relates in general to semiconductor devices, and in particular to backward diodes useful in radio frequency detection and mixing.
(2) Description of Related Art
The tunnel diode is a well-known semiconductor device that conventionally includes two regions of heavily doped semiconductor material of opposite conductivity types, separated by a relatively thin junction which permits charge carriers to tunnel through upon the application of a suitable operating potential to the semiconductor regions. The p-doped and n-doped regions of tunnel diodes are so heavily doped that they are degenerate. At equilibrium, a portion of the valence band in the p-doped region of the diode is empty and a portion of the conduction band in the n-doped region is filled.
A slight forward bias brings some levels of the filled portion of the conduction band of the n-doped region into energetic alignment with empty levels of the valence band of the p-doped region. In this situation, quantum-mechanical tunneling allows electrons to flow from the n-doped region to the p-doped region, giving a positive current that first increases with increasing bias. When the filled part of the conduction band of the n-doped region is maximally aligned with the empty part of the valence band of the p-doped region, the current flow is maximized. Subsequently, the current decreases with increasing forward bias, and approaches a minimum value when the filled part of the conduction band of the n-doped region lies opposite the energy gap of the p-doped region. When a yet larger forward bias occurs, electrons and holes are injected over the barrier between the p-doped and n-doped regions, resulting in a rapid increase in current for increasing forward bias. Thus, the current-voltage has a negative differential conductance in the forward region of its characteristic curve.
Use of a heterostructure consisting of adjoining regions of GaSb1-yAsy and In1-xGaxAs interfaced with a tunneling junction is described in U.S. Pat. No. 4,198,644 entitled, “Tunnel Diode,” issued to Leo Esaki on Apr. 15, 1980 (hereinafter the “Esaki '644 patent”). The heterostructure presented in the Esaki '644 patent includes first and second layers of Group III-V compound semiconductor alloys wherein the first layer is an alloy including a first Group III material and a first Group V material, and the second layer is an alloy including a second Group III material different from the first Group III material and a second Group V material different from the first Group V material, and wherein the valence band of the first alloy is closer to the conduction band of the second alloy than it is to the valence band of the second alloy. The preferred embodiment identified In as the first Group III material, As as the first Group V material, Ga as the second Group III material, and Sb as the second Group V material.
Also, U.S. Pat. No. 4,371,884 entitled, “InAs-GaSb Tunnel Diode,” also issued to Leo Esaki (hereinafter the “Esaki '844 patent”), provides for a tunnel diode requiring no heavy doping, and which can be readily fabricated using the process of molecular beam epitaxy. The Esaki '884 patent's tunnel diode heterostructure comprises first and second accumulation regions of relatively lightly-doped group III-V compounds, specifically consisting of In1-xGaxAs and GaSb1-y Asy, where concentrations expressed in terms of x and y are preferably zero but less than 0.3, and where the improvement consists of an interface of a relatively thin layer of a quaternary compound whose constituent materials are those of the adjoining regions. This interface provides a tunneling junction as opposed to an ohmic junction between contiguous regions of InAs and GaSb.
A need exists in the art to improve current tunneling diodes so that they may be applied to higher bandwidths, with greater dynamic range and greater sensitivity for radio frequency detection. In particular, it is desirable to provide a high degree of non-linearity near zero bias. This is in contrast to the inventions discussed above, which are designed to provide a negative resistance region near zero bias.